The root deflector has a split collar for installation around a root ball. The collar has a peripheral skirt for location beneath ground level which skirt has on its interior face guide faces on internal projections which direct root growth impinging thereon downwardly. The ends of the collar at the split line may be secured together by a snap engaging clip and the collar has a shoulder for location at ground level and over which a lawn mower may run for cutting grass adjacent to the collar. The collar may be formed of molded plastics.